Method and apparatus for precision sizing and joining of large diameter tubes



Nov. 17, 1970 J s w M ETAL 3,540,250

METHOD AND APPARATUS FOR PRECISION SIZING AND JOINING OF LARGE DIAMETERTUBES Original Filed May 8, 1967 6 Sheets-Sheet 1 FIG. 2-

INVENT R.

J D BENNIGHT A TTORNE Y8 ROBERT J4 SCHWINQHAWJR NOV. 17, 1970 sc N METAL 3,540,250

METHOD AND APPARATUS FOR PRECISION SIZING AND JOINING OF LARGE DIAMETERTUBES 6 Sheets-Sheet 2 Original Filed May a, 1967 FIG.4

INVENTOR.($)

R E M NW A w m a. N am m m a on R V Nov. 17, 1970 .J. SCHWINGHAMER ETAL250 METHOD AND ARATUS FOR PRECISION SIZING AND JOINI OF LARGE DIAMETERTUBES 1 Original Filed May a, 1967 s Sheets-Sheet 3 LZ////////f'//J///f/] FIG. '6

-so I 30 f 26 FIG.7

|/ev'- -|--|/e v-- 3/4v y INV TORS ROBER J. SCH I GHAMER BY J D BENNIGHTFIG.8

, ATTORNEYS Nov. 17, 1970 SCHWmG'HAMER ETAL' 3,540,250

METHOD AND APPARATUS FOR PRECISION SIZING AND JOINlNG 0F LARGE DIAMETERTUBES Original Filed May 8, 1967 I 6 Sheets-Sheet 4 INVENTOR.($) ROBERTJ. SCHWINGHAMER J 0 BENNIG r Nov. 17, 1970 R J. SCHWINGHAMER ETAL3,540,250

ATUS FOR PRECISION SIZING AND JOINING OF LARGE DIAMETER TUBES METHOD ANDAPP-AR Original Filed May 8, 1967 6 Sheets-Sheet 5 luvEurbRrs) F G.

ROBERT J: SCHWiNGHAMER BY J D BENNIGHT A TTORNE YS Nov. 17, 1970Original Filed May a, 1967 OF LARGE D J. SCHWINGHAMER ET AL METHOD ANDAPPAR ATUS FOR PRECISION SIZING AND JOINING IAMETER TUBES 6 Sheets-Sheet6 F l G. I2

F I G. l3 INVENTOR.($)

ROBERT J.

SCHWINGHAMER BY J D BENNIGHT United States Patent O Int. Cl. B21d 26/14US. Cl. 72-56 7 Claims ABSTRACT OF THE DISCLOSURE A method and apparatusfor portable high precision magnetomotive bulging, constricting, andjoining of large diameter tubes. The method allows decremental, veryaccurate changing of the diameter of very large tubes, as well as highquality joints obtained by either bulging or constricting overlappingends of two tubes. The apparatus consists of a magnetomotive coilpositioned either inside or outside of the tube and a non-conductingmandrel (or forming die) on the other side. The magnetomotive coil hassquare conductors which are recessed, and thus separated by an air gap,from the tube. The constricting coil has a split metal sleeve forwithstanding hoop stress. The tube (workpiece) is insulated from thecoil by a thin plastic sleeve. The power supply is an electricallyfloating system.

BACKGROUND OF THE INVENTION This is a division of application Ser. No.637,882, filed May 8, 1967.

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

Field of the invention This invention relates to a method and apparatusfor metal working and more specificially to a method and apparatus forportable high precision magnetomotive bulging, constricting, and joiningof large diameter metal tubes.

Description of the prior art In recent years, particularly, much efforthas been devoted to seeking better tools and techniques for workingmetals to the shapes required. This working of metal to desired shapesinvolves a variety of operations, such as sizing, blanking, flaring,stress removal, and other operations which are well known in the art.Among the newly developed tools and techniques for performing metalworking operations are those related to the sphere of activity known aselectromagnetic forming or magnetomotive forming. This mode ofmanipulating metal is based on the creation by electrical discharge ofan intense magnetic field about a shaped conductor, such as a coildisposed adjacent to the metal workpiece. An induced current is thuscaused to flow in the workpiece in a direction opposite to that flowingin the coil. The field associated with this induced current reactsagainst the magnetic field around the coil producing intense forcesbetween the coil and the metallic workpiece. If the coil has a degree ofstructural or inertialrigidity greater than the workpiece, a yielding ofthe workpiece under the magnetic forces will occur.

Working of metal with electromagnetic forces has several intrinsic andimportant advantages. The iso- 3,540,250 Patented Nov. 17, 1970 dynamicforces are distributed relatively uniformly through the material whichis being manipulated, effecting a natural reshaping without causingappreciable change in the grain structure of the material. Very highstrain rates may be achieved, afi'ording heretofore impossibleaccomplishments in the forming of hardened materials. Surface marring ofthe workpiece, a bothersome aspect of more conventional formingtechniques, can be avoided in magnetomotive forming. The entireoperation is clean, dry, easy to execute, and may be performed withapparatus essentially free of moving parts.

Although prior art devices and methods for magnetomotive forming offerinherent advantages in comparison to more conventional practices,relatively few tools suitable for specific job situations andapplications have been introduced. Many times, it is impractical orimpossible to bring the workpiece to the metal forming equipment. Forexample, in the fabrication of large metallic structures, such as rocketvehicles, sizing, blanking, and stress removal operations are frequentlyrequired. Also, the great size and weight of the components prohibitmovement and application of the components to the forming equipment.Therefore, a fully portable device is needed which not only can bereadily moved into proximity to the workpiece but which can be applieddirectly to the particular workpiece area that is to be manipulated.

One particular problem which plagues engineers who design and fabricatelarge aerospace rockets is the difliculty in obtaining large metal tubeswhich are manufactured to close enough tolerances. Large rocket enginesuse liquid oxygen at a rate of several tons per second, fed throughlarge metal liquid oxygen lines. These lines pass down through the fueltank and are covered by an oversize aluminum alloy tunnel which allowsan insulating gap between the fuel line and the tunnel. Particulardifiiculties have been encountered in obtaining tunnels of close enoughtolerance to insure their successful welding into mating fittings in thebulkheads or tank ends. Therefore, a method of precision sizing wasneeded to bring the large tubes into usable tolerance.

The accomplishment of bulging large metal tubes as practiced in theprior art involves very high hydraulic pressures plus a requirement formassive end loading forces. The equipment is not only extremely massivebut quite expensive as well. In operation it is clumsy, cumbersome, andrelatively complicated.

Constriction of large tubes as taught by the prior art is accomplishedby one of two methods:

(1) The tube is made undersized and an expanding mandrel is used tobring the tube to the required diameter by exerting pressure on theinside of the tube; or

(2) The oversized tubes are worked back down to the proper diameter by arepetitive impaction on the tube by a set of guillotine dies.

Both the hydraulic bulging and constricting prior art methods justdescribed have the following disadvantages:

(1) Fluids and high pressures are frequently used and leaks, safetymessiness, and precision control are always a problem.

(2) Work hardening is frequently a serious problem in conventionalbulging or constricting Work; therefore, heat treating operations arefrequently required to obtain the desired level of strength.

(3) Forming takes place at such low energy rate levels that actualmaterial rupture is a constant worry; conventional formability is therule.

(4) The distribution of the forming force is planar and laminar andcertainly non-isodynamic.

(5) Strain rates attained are quite low relative to the magnetomotivemethod and formability is in no way comparable with the magnetomotivetechnique, Where high strain rates allow forming of high strength alloysin the hardened condition.

(6) The equipment required for bulging or constricting of large tubes isby no means portable. Rather, it is massive and non-mobile, with verylittle flexibility.

As has been discussed prevously, the concept of magnetomotive metalforming is known in the prior art. However, the prior art magnetomotivedevices have been limited to the forming of small objects, using smallcoils. These magnetomotive devices previously proposed suifer from anumber of drawbacks. Coil designs in particular in the prior art devicesare not capable of handling the voltages and currents necessary to formthe large diameter tubes. In actual tests, these coils have deterioratedquite rapidly in use. The quantity of energy delivered by them wasdeficient, variable, and unpredictable from one discharge to the next.The occurrence of electrical arcing from the coil to the workpiece couldnot be tolerated because of the attendant danger of marring ordestruction of the workpiece. These objectionable features of the priorart devices have been found to be largely attributable to their design,particularly their coil design, and to the selection and arrangement ofthe materials in the coil.

Some of the prior art magnetomotive devices heat the workpiece beforeforming it. This has the disadvantage of allowing the workpiece todistort by relieving residual stresses. Moreover, heating the workpiecemay remove the temper of the metal or otherwise adversely affect thecharacter of the metal in the workpiece, and except for certainrefractory metals, is generally not desirable when forming in thehardened high strength condition, as the present invention allows.

Lastly, it has been noted that the method of employment of the prior artdevices was not suitable for use with large metal tubes because of thepatricular problems involved and the inability to cope with the veryhigh power required.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a method and apparatus for portable sizing and joining oflarge diameter metal tubes which allows greater precision and accuracythan with prior art devices.

It is a further object of the invention to provide a method andapparatus for sizing and joining large metal tubes which allows workingdirectly with the hardened, high strength materials, thus eliminatingwelding, heat treating, and distortion.

It is, a still further object of this invention to provide a method andapparatus which allows successful magnetomotive forming of very largemetal tubes using very large coils, without coil breakdown.

These and other objects are accomplished in the present invention inwhich an electrical circuit having a bank of fast discharge typecapacitors is connected by means of a switch to a magnetomotive coilthrough low inductance cables. The coil has a plurality of turns whichare recessed into the coil form so as to create a specially calculatedair gap between the conductors and the tube to be sized. Application ofpower to the circuit causes pressure of the order of 840 pounds persquare inch which, for example, is suflicient to affect high precisionsizing of a 25 inch diameter, .224 inch thickness wall cylinder of highstrength aluminum alloy to as much as a .100 inch diameter reduction.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fullyunderstood when taken together with the accompanying drawings in which:

FIG. 1 is a flow diagram showing the components of an illustrativeembodiment of the invention used for constricting a large diameter tube.

FIG. 2 is a perspective view showing a constricting GOil and mandrel inposition on a large diameter tube,

FIG. 3 is a side elevational view of a constricting coil.

FIG. 4 is an edge view of the constricting coil of FIG. 3.

FIG. 5 is a sectional view of the constricting coil taken along line 55of FIG. 3.

FIG. 6 is a sectional view of the constricting coil in position on alarge diameter tube.

FIG. 7 is a greatly enlarged sectional view of a constricting coil,showing a possible arrangement of the coil.

FIG. 8 is a sectional view illustrating a voltage breakdown path in themagnetomotive sizing coil incorporated into the invention.

FIG. 9 is an end view of a large thin-walled tube with a bulging coilinside the tube and a shaping die outside the tube.

FIG. 10 is an edge view of a bulging coil.

FIG. 11 is a side elevational view of the bulging coil of FIG. 10.

FIG. 12 is a sectional view showing a bulging coil and a shaping die inposition on a large thin-walled tube.

FIG. 13 is a sectional view showing an arrangement for joining largethin-walled tubes.

DESCRIPTION OF THE PREFERRED {EMBODIMENTS With continued reference tothe accompanying figures, and with initial attention directed to FIG. 1,reference number 10 generally designates an illustrative embodiment ofthe invention used for reduction sizing of large thin-walled metaltubing. A principal component of the invention is the shrinking coil 12.A portable power supply is provided for supplying and controlling pulsesof electrical energy so as to produce a varying field of high intensityabout the coil 12. This power supply includes a high voltage source 14for charging a bank of capacitors 16. Connected to the capacitor bank 16is a switching means 18, such as an ignitron. The switching means 18 isoperated by a trigger 20.The electrical energy system just described,which has an energy capability of about 240 kilojoules, is connected tothe coil 12 by a plurality of transmission lines 22, preferably co-axialcables, which enter the electrical energy system through a connector box24. It is desirable that the transmission lines 22 be quite long so thatthe coil 12 may be taken a considerable distance from the power supplyto the location of the workpiece 26.

The entire system is electrically floating, which prevents the settingup of high voltage stresses between the work coil and the workpiece.This also prevents the dangerous situation which is encountered when oneuses a grounded system, and the workpiece is actually poorly grounded.This can be lethal, because of the high potentials which result fromhigh current flowing through a poor (high resistance) ground.

Considerations behind the design of constricting coil 12 are as follows:I

It has been determined in plasma physics work that magnetic fieldpressure follows the basic relationship:

Where P is the pressure in newtons per square meter, B is the magneticfield strength in Teslas, and U: 1.25 X 10* in air in the mks. system ofunits.

From this, it is easily shown that one megagauss Teslas) is roughlyequivalent to 580,000 p.s.i. of actual pressure. This is generallyindicative of the tremendous pressures attainable by the magnetic fieldtechniques.

The following example, it is assumed that a 25" diameter, .224" wallthickness cylinder of aluminum alloy is to be precision constricted.100" in diameter. For such a cylinder, the longitudinal yield is 47,000p.s.i. and the transverse yield is 46,000 p.s.i. However, hoop stresseffects come into play quite beneficially, so that the actual magneticpressure required to constrict the cylinder by exceeding the yield isconsiderably less than the above values. This is calculated as follows:

where Pm is the magnetic pressure in p.s.i. required to exceed theyield, S is the hoop stress equivalent of the yield (47,000 p.s.i.), tis the cylinder wall thickness, and d is the tunnel diameter.

Therefore, 840 p.s.i. magnetic pressure will cause yielding and anymagnetic pressure in excess will be sure to produce gOOd forming. Apressure of 840 p.s.i. is equivalent to 5.82 l newtons per square meter.A coil configuration is then calculated, based on experience, whichstrikes a reasonable balance between area to be worked, and inherentinductance. Then, the magnetic field strength required to attain 840p.s.i. (5.82 l0 newtons per square meter) pressure is computed asfollows:

:840 p.s.i.

This is the value of peak field strength which will cause yielding ofthe material. To attain that field, one must now consider the currentrequired to do this. It has been determined empirically by the inventorsthat the following relationship holds:

IL1/2 w Where E is the capacitor charge voltage, L is the sys teminductance under loaded conditions, C is the capacitance of the bank,and I is the current in amperes.

Therefore, the design data has produced the following information:

(1) Field required (considering hoop stress): 3.8 Tesla (2) Current toattain 3.8 Tesla: 64,000 amperes (3) Voltage needed on a 1200 ,uf.capacitor bank to meet above requirements: 8.5 kilovolts Tests andapplication have shown this method reliable in determining the thresholdof forming under conditions in which constricting coil 12 is employed.The energy required then, to just cause yielding of the material is:

Q= /2 CE 43,400 joules Where Q is energy in joules, C is capacitance infarads and E is voltage in volts.

Referring now to FIG. 2, the portable constricting coil 12 may be seenin position on the workpiece, large, thinwalled tube 26, An undersizedmandrel 28 is positioned on the inside of the tube 26, opposite the coil12. Between the coil 12 and the tube 26 is positioned a thin plasticsleeve 30. This sleeve, which may, for example, be on the order of .007inch thick, helps prevent arcing between the coil and the workpiece, andtherefore prevents the work piece from becoming a host conductor. Thecoil 12 is suspended from the hook 32 of a hoist (not shown). The hook32 is fastened into the lifting hole 34 of weight suspension plate 36.Side holes 38 in plate 36 connect with suspension cables 40 having eyefittings 42 at either end. Fittings 42 connect with a pair of eyebolts44, mounted on opposite sides of the coil 12. Each eyebolt 44 isfastened to a pair of brackets 46 but is electrically insulated from oneof the two brackets 46 by an insulator 48, as will be explained below.

FIGS. 3, 4, and 5 show the details of the constricting coil 12. As maybe seen best in FIGS. 3 and 5, conductor 50 is continuously wound on theinside of the coil form 52 and its turns are recessed considerably fromthe inside diameter 54 of the coil form 52, for reasons which will beexplained below, Conductor 50 has two lead portions 56 which connect tocoil conductor terminals 58 located on either side of coil form 52. Inorder to prevent leads 56 from cutting coil form 52 under the greatforces of radial stress during forming, leads 56 are arranged on arounded path from the inside diameter 54 of the coil form 52 to theterminals 58. Terminals 58 are firmly fastened to coil form 52, which isalso for the purpose of preventing shearing of the coil form because ofradial stress. For the purpose of preventing shearing of the coil formbecause of axial stress, stainless steel bolts 60 extend laterallythrough the coil form 52. Severe hoop tension provides additionaltensile and shear resistance.

As may be seen in FIG. 4, conductor 50 makes four full turns around theinside diameter 54 of the coil form 52. Square wire is used forconductor 50 because this gives much more bearing surface in the axialdirection, and the turns then do not shear off the coil form lands 61due to the tremendous axial compressive forces during discharge. Coilconductor 50 is preferably made of any known material which provides anadvantageous tradeoff between good tensile strength and reasonableelectrical conductivity. A layer of tough insulating enamel or varnish62 is used over the conductor 50 to provide additional protectionagainst voltage breakdown.

Looking again at FIGS. 3 and 5, two stainless steel ring segments 64extend around the outer circumference of the coil form 52 and form aring which is designed to contain, or withstand, the hoop stressdeveloped during constricting operations. Each of the two ring segments64 has a pair of brackets 46 mounted at its ends. As mentioned above,brackets 46 of adjacent ring seg ments 64 are fastened together, on bothsides of the ring, by eyebolts 44. Eyebolts 44 are used for suspendingthe coil 12. In addition, they space apart, as well as fasten together,the adjacent brackets 46. Two air spaces 66 sep arate the two ringsegments 64. As also mentioned above, an insulator 48 is used toelectrically insulate each eyebolt 44 from one of the two adjacentbrackets 46. Thus, the two ring segments 64 are electrically insulatedfrom each other, even though they are securely fastened together. Inthis way, the coil structure 12 avoids the shorted turn effect, whichwould certainly appear as a one-turn secondary of a transformer to thework coil 50. Stainless steel is preferred for use in the ring segmentsbecause it does not extract much energy from the coil and does notcreate high repulsion forces. This is because of its high resistivityand the deep penetration of the magnetic field in stainless steel.

FIG. 6 shows a sectional view of the constricting coil 12 mounted on atube 26 with non-conducting mandrel 28 on the inside of the tube 26. Airgap 68 may be seen between the conductors 50 and the tube 26. Also,between the coil 12 and tube 26 is the plastic sheet 30, which waspreviously described with regard to FIG. 2.

Air gap 68 is an important feature in the design of the coil 12 of thepresent invention. In the recess-air-dielectric concept which isemployed in the invention, it is emphasized that the rectangular coilconductors 30 are recessed by a specific amount, which is rathercritical for optimum results.

The rationale behind the coil design is as follows:

Looking at FIG. 7, a greatly enlarged sketch may be seen showing apossible arrangement for the coil 12 and workpiece (tube) 26. If oneattempts to provide solid insulation 70 over the conductors 50, analmost impossible situation results. This is because the voltage acrosstwo dielectric materials (essentially in series) divides in a mannerinversely proportional to the dielectric constant. A typical voltagebreakdown path is shown at 72. This path passes from conductor 50through insulators 62 and 70, air gap 68, insulator 30, a portion of thworkpiece 26, insulator 30, air gap 68, insulators 70 and 62, andfinally back to an adjacent turn of conductor 50.

FIG. 8 is a diagram showing a straight-line breakdown path 74 which maybe substituted for the indirect voltage breakdown path 72 shown in FIG.7, in order to simplify the theorectical considerations involved involtage breakdown. Assuming that: (1),. the insulation has an dielectricconstant of 3, and (2) the electrical system is floating, then thevoltage V divides as shown in the diagram with A; v. across each of thetwo insulator portions.

This means that the turn-to-turn voltage gradient will appear across theair gap. Values one might ordinarily assume for insulation thickness andair gap could be A; inch and perhaps as little as .003 inch,respectively. If the total voltage drop across the insulation is A v.,then the voltage drop across each of the two portions of insulatingmaterial is /s v. If the coil is a four-turn, 10 kilovolt coil, then Thedielectric stress (volts/inch) for both pieces of insulating material inthe breakdown path of a 10 kv., four-turn coil is then:

kv. The dielectric stress across the one piece of insulation will thenbe:

V =2.5 kv

stress (insulator) stress (air)= =312,000 volts/inch M v.) (10 volts)inch =20,000 volts/inch or 20 volts/mil (which is still completely safefor the insulation) However, the stress in the air gap gets even higher:

v. (10 volts) 7500 V.

=2,500,000 volts/inch stress (insulation) stress (air) (a value which isdangerous, but represents over-kill in a sense, since the lower airstress obtained with the ungrounded workpiece would already causetrouble.)

Obiously, then, one must arrive at the proper proportion of air gapversus solid dielectric material to avoid serious breakdown problems.The solution to adequate air gap protection may be calculated asfollows:

1 inch 3 gap X Volts X 76,200 volts =.0328 inch This means that a .0328inch gap would barely stand off the voltage. For safety, this is usuallydoubled. When this is done, voltage breakdown problems are virtuallyeliminated. This causes some reduced coil efifciency because of poorercoupling but this is compensated for by the fact that the field profileis much more constant at that distance, and the coil then is much lesssensitive to coupling and energy content variations.

THICKNESS LIQUID OXYGEN TUNNEL WITH SUBSE- QUENT PASSES OF FORMING COILOutside diameter (inches) Reduction Measured- (inches) Before Pass #1.After Pass #1 After Pass #2 After Pass #3. After Pass #4...

Thus, it may be seen that the constructing coil of the present inventionsolves the very serious design problem of voltage breakdown inherent inlarge coils. In addition, it provides, as a nauxiliary benefit, muchmore accurate results than were possible before. Moreover, in spite ofrepeated sizing shots, very little work hardening results.

Referring back to FIG. 1, one cycle of operation of the inventionoperating in the constricting mode follows: Power supply 14 furnishespower to capacitor bank 16, where the power is stored. When trigger unit20 closes ignition switch 18, power from capacitor bank 14 is switchedinto connector box 24 and then through coaxial transmission lines 22 toconstricting coil 12. When coil 12 receives the power pulse, an intensemagnetic field is set up about the coil 12. Also, a current is inducedin the workpiece 26. This induced current flows in a direction oppositeto the direction of the current flowing in the coil 12. The magneticfield around the workpiece 26 reacts against the magnetic field aroundthe coil 12, producing intense forces between the coil 12 and theworkpiece 26. Since the coil 12 has a greater structural rigidity thanthe workpiece 26, a yielding of the workpiece occurs.

Ditferent modes of operation of the constricting device are possible.For example, the coil 12 may be pulsed several times in position, orcoil 12 may be pulsed and moved incrementally along the surface of thetube 26. In either mode, successive pulses yield a decremental decreasein the diameter of the tube 26. Therefore, after several passes ineither mode, very fine precision sizing of the tube 26 may beaccomplished.

FIG. 9 shows an illustrative embodiment of the invention which may beused for expanding or bulging a large tube 26. A bulging coil 76 ispositioned on the inside of the tube 26. Conductors 78 of the bulgingcoil are wound on the outside of the coil form 80 of the bulging coil76. Outside forming die 82 is shown in a slightly open con dition, in aposition opposite the bulging coil 76. Die 82 is equipped with a hinge84 and a pair of locking brackets 86. Die 82 also has a concave portion88 which allows room for the tube 26 to be expanded by the bulging coil76. When forming die 82 is closed and the bulging coil 76 is pulsed,tube 26 is bulged outward in a convex configuration (see 90 in FIG. 12)which conforms to the shape of the concave portion 88 of die 82.

FIGS. 10 and 11 show the details of the portable bulging coil 76.Conductor 78 is wound on the outside of the coil form 80 and is recessedconsiderably from the outside diameter 92 of the coil form 80, for thesame reasons already discussed for the constricting coil 12. Coilconductor 78 has lead portions 94 and 95, which lead out from theoutside of coil form 80 to the inside of the coil form 80. As may beseen best in FIG. 10, the lead-out path of lead portions 94 and 95 isrounded, to prevent cutting of coil form 80 by axial stress applied bycoil conductor 78, during forming. Lead portions 94 and 95 connect toco-axial cables 22 and the remaining elements of the power supplyalready described with respect to FIG. 1.

As may also be seen in FIG. 10, coil conductor 78 makes four full turnsaround the outside of coil form 80. Square conductors are used in thebulging coil 76 for the same reason they were used in the constrictingcoil 12, namely, to prevent shearing off the coil form lands 96, whichare located between the turns of conductor 78. Conductor 78 ispreferably made of any known material which has good tensile strengthand reasonable electrical conductivity. A layer of insulating enamel orvarnish 98 is used over the conductor 78 to provide additionalprotection against voltage breakdown.

A stainless steel split ring, such as ring 64 previously described as apart of the constricting coil 12, is not necessary in the design of thebulging coil 76. Since the conductors 78 in the bulging coil 76 arelocated on the outside portion of coil form 80, hoop stress (tendency ofthe coil to break apart in a radial direction) is not a problem.

Air gap 100 is an important part of the design of the bulging coil 76.Without proper design, a large coil will fail repeatedly. As explainedmore fully with respect to the constricting coil 12, providing an airgap of proper dimension does reduce coupling efliciency but produces thebeneficial and unexpected result of making the field profile much moreconstant and therefor making the coil 76 much less sensitive to couplingand energy content variations.

FIG. 12 shows a sectional View of a tube 26 which has been bulgedoutwardly so as to form a convex portion 90 on the outside surface ofthe tube 26. Bulging coil 76 may be seen in position on the inside ofthe tube 26. Outside die 82 with concave portion 88 is positioned on theoutside of the tube 26, directly opposite bulging coil 76. Plasticsleeve 102 is positioned on the inside surface of the tube 26 to preventarcing between the coil 76 and the tube 26.

Referring first to FIG. 12, one cycle of operation of the embodiment ofthe invention suitable for bulging large diameter tubes follows: bulgingcoil 76 is positioned inside the tube 26 at the point where the tube 26is to be bulged. Outside die 82 is positioned opposite the coil 76 andis closed. Referring now to FIG. 1, where the elements of the powersupply may be seen, power source 14 furnishes power to capacitor bank16, where the power is stored. When trigger unit closes ignitron switch18', power from capacitor bank 14 is switched into connector box 24 andthen through co-axial transmission lines 22 to bulging coil 76 (FIG.12). As may be seen in FIG. 12, when coil 76 receives the power pulse,tube 26 is bulged outward and tends to conform to the concave innershape 88 of the outside die 82. In order to achieve the amount ofbulging desired, the bulging coil 76 may be pulsed several times inposition or it may be pulsed and moved incrementally along the surfaceof the tube 26. Successive pulses yield a decremental increase in thediameter of the tube 26. After several passes, very fine precisionsizing of the tube 26 may be accomplished.

FIG. 13 shows another embodiment of the invention, which is used forjoining together two large tubes 26 and 104. To perform the joiningoperation, the two large tubes 26 and 104, which have slightly differentdiameters, are arranged with their end portions overlapping. A bulgingcoil 76, such as has already been described with reference to FIGS. 9 to12, is positioned inside the smaller tube 104. A shaping die 106 havinga plurality of concave portions 108 is arranged on the outside of theouter tube 26 at a point opposite the bulging coil. A plastic sleeve 102is arranged on the inside surface of tube 104 so as to insulate thebulging coil 76 from the tube 104. When the bulging coil 76 is pulsed ina manner already described for the other embodiments of the invention,both tubes 26 and 104 are bulged outwardly against forming die 106.Tubes 26 and 104 are then crimped together. Their overlapping portionshave a corrugated configuration, with convex portions caused by thetubes 26 and 104 10 being forced against the concave portions '108 offorming die 106.

The coil 76 may be pulsed more than once in place if necessary toproperly join tubes 26 and 104. Also, coil 76 may be moved incrementallyand pulsed, if necessary to create a longer crimped overlap portion ofthe tubes 26 and 104. However, the outside forming die 106 will normallyremain stationary, in joining operations, whether or not the bulgingcoil 76 is moved.

Another method of joining the overlapping end portions of two largethin-walled tubes is by using a constricting coil outside the tubes anda special mandrel inside the tubes. However, this method is perhaps lessconvenient than the method and apparatus for joining which is disclosedabove. This is because of the fact that, after the joining operation iscompleted, difficulty is experienced in getting the mandrel out of thetubes. This problem could -be solved, of course, by constructing aspecial mandrel which: (1) may be disassembled or (2) is cut diagonallyacross its circumference, so its diameter may be reduced, for removal.

From the foregoing it may be seen that the applicants have inventedno'vel methods for constricting, bulging, and joining large thin-walledtubes and novel apparatus for carrying out each of the methods. Theinvention allows a fantastically high strain rate which, coupled withthe isodynamic nature of the applied force, results in optimumformability without surface marring of the workpiece, and permitsworking in hard materials directly. The operation is quite fast in allinstances, since pulse duration s only a few hundred micro-seconds andhigh repetition rates can be obtained. The equipment is lightweight andportable, yet strong enough to withstand the tremendous stresses presentin equipment performing a sizing job of this magnitude. It is easy toachieve very fine precision sizing, even in very large diameter tubes orcylinders, either in bulging or constricting. If tolerances of the orderof .020 inch to .030 inch are acceptable, than no mandrels or dies areneeded, and free forming is quite satisfactory.

Obviously, numerous modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that, within the scope of the attendantclaims, the invention may be practiced otherwise than is specificallydescribed.

What is claimed is:

1. A portable magnetomotive coil for high precision constricting largediameter metal tubes comprising:

(a) a coil form for enclosing a portion of a tube to be constricted,said coil form having a circular inner surface; said inner surfacehaving a center portion, the surface of said center portion beingrecessed from the said inner surface, said recessed surface of saidcenter portion having a recessed continuous groove arranged in a helixaround it, a land separating the adjacent turns of said groove,

-(b) a conductor having a plurality of turns, said conductor being woundin the said groove in the surface of said center recessed portion, saidland separating the adjacent turns of said conductor,

(c) whereby the turns of said conductor are separated from the tube tobe constricted by a fixed air gap.

. 2. The portable magnetomotive coil for constricting large diametertubes as set forth in claim 1 further comprising a plurality of boltsextending laterally through said coil form for withstanding axialstress.

3. The portable magnetomotive coil for constricting large diameter tubesas set forth in claim 2 further comprising a split metal sleeve mountedon the coil form for withstanding hoop stress, said sleeve comprising atleast two segments fastened together and electrically insulated fromeach other to prevent said sleeve from acting as a one-turn secondarytransformer winding.

4. The portable magnetomotive coil for constricting 11 a large diametertubes as set forth in claim 3 wherein the cross sectional area of thesaid conductor is square in order to provide additional bearing surfacein the axial direction and thus prevent shearing of said coil form landsunder axial compression force.

5. A portable magnetomotive coil for high precision bulging of largediameter metal tubes comprising:

(a) a coil form positioned in a portion of a tube to be bulged, saidcoil form having a circular outer surface, said outer surface having arecessed center portion enclosed by two flanges located at the edges ofsaid outer surface, said recessed center portion having a recessedcontinuous groove cut in a helix, around it, a land separating theadjacent turns of said groove,

(b) a conductor having a plurality of turns, said conductor being woundin the said groove in the surface of said center recessed portion, saidland separating the adjacent turns of said conductor,

(c) whereby the turns of said conductor are separated from the tube tobe bulged by a fixed air gap.

6. The portable magnetomotive coil for bulging tubes 12 as set forth inclaim 5 further comprising a plurality of bolts extending laterallythrough said coil form for withstanding axial stress.

7. The portable magnetomotive coil for bulging tubes as set forth inclaim 6 wherein the cross sectional area of the said conductor is squarein order to provide additional bearing surface in the axial directionand thus prevent shearing of said coil form lands under axialcompression forces.

References Cited UNITED STATES PATENTS 3,282,077 11/1966 Brower et a17256 3,288,006 '11/1966 Erlandson 7256 3,312,093 4/1967 Brower 72--563,345,844 10/1967 Jansen et al. 7256 3,372,566 3/1968 Schenk et al. 72563,380,271 4/1968 Habdas 7256 RICHARD J. HERBST, Primary Examiner US. Cl.X.R. 72706; 336208

